Velocity Measurement - US EPA Method 2 is performed to determine the average stack gas velocity from measurement of the gas density and average velocity head using Type "S" (Stausscheibe or reverse type) pitot tube. Apex Instruments manufactures and guarantees Type "S" pitot tubes in accordance with the Method 2 design specifications. A variety of materials, sizes and configurations are available. The most common are constructed from either 1/4 inch or 3/8 inch OD Stainless Steel or Alloy 600 tubing. Apex Instruments offers calibration equipment and services.